Not applicable.
Not applicable.
The present invention generally relates to medical diagnostic imaging and in particular to a method and apparatus for adjusting the dynamic range of a digital medical image to be presented.
X-ray imaging has long been an accepted medical diagnostic tool. X-ray imaging systems are commonly used to capture, as examples, thoracic, cervical, spinal, cranial and abdominal images that often include the information necessary for a doctor to make an accurate diagnosis. When having a thoracic x-ray image taken, for example, a patient stands with his or her chest against an x-ray sensor as an x-ray technologist positions the x-ray sensor and an x-ray source at an appropriate height. The x-ray sensor then detects the x-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system scans out the detected x-ray energy and prepares a corresponding diagnostic image on a display. Optionally, the x-ray sensor may be a solid state digital image detector. If the x-ray sensor is a conventional screen/film configuration, the screen converts the x-rays to light, to which the film is exposed.
In conventional radiographic imaging systems, the x-ray technique is chosen by the operator. An operator or an automatic exposure control system selects or determines a desired exposure for the selected screen/film configuration in order to obtain a desired optical density of the exposed film. The optical density represents the xe2x80x9clightnessxe2x80x9d or xe2x80x9cdarknessxe2x80x9d of the resulting film once exposed to x-rays. By controlling the manner (e.g., time, orientation, etc.) of exposure of the detector or screen/film to x-rays, the film lightness or darkness is varied. It is preferable to achieve a consistent optical density from one exposure to the next in order to facilitate diagnosis and examination by physicians when analyzing radiographic images. A screen/film combination system does not have a linear response to x-ray exposure, but instead is non-linear and defined by an x-ray sensitometry curve of the particular screen/film combination. The sensitometry curve relates input exposure to a resulting optical density. The non-linear response of a screen/film combination affords a desirable image contrast at exposure levels falling in the middle of the exposure range. The non-linear response compresses image contrast outside of the middle range. The dynamic range of a screen/film combination system is xe2x80x9cfixedxe2x80x9d. The operator (or via automatic exposure control system) attempts to regulate the transmitted x-ray exposure of the diagnostic regions to fall within the limited middle dynamic range of the screen/film. Different exposures arise from one patient to the next, from one film type to the next, from one medical imaging system to the next, from one orientation to the next and the like.
In the past, it has been quite difficult to maintain a consistent optical density from one exposure to the next (e.g., patient to patient, film to film, system to system, patient angle to patient angle) due to inherent differences. For instance, each patient has a slightly different size and anatomy which causes the internal organs of the patient to have different x-ray attenuation and may be located at different positions relative to the detector or screen/film. For example, when attempting to obtain an x-ray of a chest image, every patient""s lungs and rib cage are of a different size. The resulting variance in x-ray attenuation creates a large variance between exposures. Variation in patient position and orientation further create variance between exposures. Variance between exposures may result in the diagnostic exposure range for a particular exposure to fall outside of the detector""s xe2x80x9cdesiredxe2x80x9d exposure range and compress the diagnostic dynamic range. The resultant optical density may become variable due to particular patient pathology, to foreign objects within a patient (e.g., pacemakers and the like), to differences in patient thickness, as well as to differences in x-ray acquisition parameters (e.g., x-ray energy, dose, exposure time and the like).
automatic exposure control is commonly used with radiographic systems in an attempt to control the exposure range and the resultant optical density of the exposed film. Automatic exposure control systems typically use an x-ray sensitive ion chamber located proximate to the detector or screen/film combination and proximate to the patient. For instance, the operator positions the patient so that his/her lungs are proximate the selected ion chamber for a chest exam. The automatic exposure control terminates the x-ray exposure when a preset dose is measured.
However, automatic exposure control systems have experienced difficulties. In particular, the exact position of an individual patient""s lung is unknown while the ion chamber position is fixed. Hence, different patients continue to create a large variance in the resulting exposure to the detector. For instance, the ion chambers may not actually be located proximate certain patient""s lungs. When an ion chamber is located proximate an anatomy other than the lung, the automatic exposure control terminates exposure based on inaccurate measurements. A certain percentage of chest films result in creation of either too dark or too light of an image. When the image is too dark or too light, it may be necessary to repeat the x-ray examination to retake the medical image. It is quite time consuming to retake medical images. Film development may require a relatively long period of time, such as five to fifteen minutes, during which the patient may leave the image acquisition area.
Further, a resulting presentation of a medical image is determined by the selection of the type of detector or film/screen configuration in combination with the desired x-ray technique. Different types of detectors and screens/film configurations experience different amounts of image contrast, different signal to noise ratios (SNR) and different dynamic ranges. In the past, SNR has been modified by varying the input exposure. However, to maintain a constant optical density as one varies the exposure level, the detector, film or screen types must be changed to match the expected dynamic range. It is quite cumbersome to change detectors, screens or films, and thus it is rarely done.
More recently, digital detectors have been proposed for use with radiographic imaging. Digital detectors afford a significantly greater dynamic range than convention screen/film configurations, typically as much as two to three times greater. Heretofore, the automatic exposure control and/or operator must still be relied upon to limit the exposure to the digital detector to account for the detectors greater, yet finite, dynamic range. The digitally detected image may be image processed to attain the expected film-like sensitometry curve. That is, the dynamic range is mapped via a contrast look-up table to achieve the desired contrast and optical densities when printed or viewed.
Moreover, with the advent of electronics and digital technology, it has become desirable to offer xe2x80x9csoft copiesxe2x80x9d of medical images to physicians for examination. Soft copies refer to the display of medical images on a television, computer screen and the like after the medical image has been image processed. The soft copy medical images, in many instances, replace a hard copy of the medical image, such as previously provided on exposed x-ray film. Hard copy x-ray films are held up to a back-lit light for examination by the physician. Electronic medical images may be routed more quickly and provide grater viewing flexibility than hard copies for an examining physician.
However, soft copy medical images have experienced certain drawbacks. For instance, the contrast or optical density of medical images when displayed electronically may significantly differ from the contrast or optical density of the medical image. Differences in contrast or optical density may be undesirable to physicians who have grown used to analyzing hard copies. It may also be desirable to maintain consistent contrast or optical density for soft copies and hard copies when physicians alternatively use both. In addition, it may be desirable for physicians to optimize the dynamic range in a subset of region(s) corresponding to particular patient anatomy. A mechanism to preset a limited number of contrast/optical density settings for quick access is needed.
Further, conventional systems using digital detectors have experienced non-ideal compression of the medical image dynamic range. Digital detectors typically store 8 to 16 bit values for each pixel location. Thus, a 14-bit detector offers an approximate dynamic range of 1 to 16,000. However, a subset of the detector""s dynamic range may only be used for the actual diagnostic range, namely the dynamic range of the output device, i.e., the monitor or digital film printer which typically support 8, 10, or 12-bit data values.
Conventional digital systems use a fixed dynamic range relation to map all detector pixel values to pixel values of a medical image. Consequently, when a digital image is formed from a detected image, the output medical image contains pixel values limited to a much smaller dynamic range than that of the detector. For instance, an image processed medical image may be limited to 8-bit pixel values, thereby affording a dynamic range of 1 to 256 gray scale values. Hence, the image processing system compresses the dynamic range of the image from a 14-bit value per pixel to an 8-bit value per pixel. Compression of the dynamic range generally results in an uneven optical density. Fixed dynamic range relations in past systems have not corrected for dynamic range compression.
A need remains for an improved dynamic range detection and control method and apparatus for use with digital medical imaging, such as in radiographic imaging.
A method and apparatus are provided according to the preferred embodiments of the present invention for controlling a dynamic range of a digital medical image for a medical diagnostic imaging system. Initially, a film type is selected along with optical densities for a particular anatomic structure. The system may store characteristics associated with multiple film types and/or multiple anatomical structures. Once a particular film type is selected, maximum and minimum optical densities therefore are obtained from memory. Next, characteristics for an original medical image are obtained representative of the dynamic range of the detected medical image using statistical methods either over the entire image or in subregions of the image. The dynamic range characteristics for the detected image, the selected film characteristics and optical densities for particular anatomic structures are used to form a xe2x80x9csensitometryxe2x80x9d model defining a relation between gray levels of the original medical image and target optical densities for a desired output presentation medical image. An inverse of the output device""s characteristic curve (gray level display function) is calculated, in order to calibrate gray levels of the presentation image which will be displayed to a user, such as through a printer or monitor. The sensitometery model is combined with the inverse function associated with the characteristic curve or gray level display function to form an auto-contrast map defining a relation between the dynamic range of the original medical image and a target dynamic range for a desired presentation image to be formed from the original medical image. The original medical image is then passed through the auto-contrast map to form a final presentation image that is presented to the user, such as via a printed film on a lightbox or monitor.
The system may utilize digital detectors or film/screen configurations to obtain the original medical image associated with a particular patient.
According to one embodiment, the gray level to optical density model is based upon predefined maximum and minimum optical densities, N selected optical densities associated with particular anatomic structures and measured gray levels obtained from the original medical image which correlate to the selected anatomic structures associated with the N optical densities. These gray levels are transformed to the log-exposure domain using a function which may be calibrated. The gray level to optical density model may represent a sigmoidal or linear curve. Optionally, the gray level to optical density model may correlate dynamic range of the original medical image to a target dynamic range based on user selected and/or measured optical densities for particular anatomic structures. A look-up table may be used to store a sensitometry curve characterizing a relation between gray level and optical density. The look-up table may be used to define the gray level to optical density model which in turn may be used to generate the auto-contrast map.
Optionally, film characteristics for a plurality of film types and/or anatomical regions may be stored, one of which may be selected when an operator chooses a desired film type. The film characteristics may include maximum and minimum optical density characteristics for an associated film type. The auto-contrast map may be stored as a look-up table.
In an alternative embodiment, the gray level to optical density model may be replaced with a localized characteristic curve when a user selects regions of interest in the medical image associated with a particular anatomic structure. The localized characteristic curve defines a relation between a portion of the dynamic range for the original medical image and an expanded target dynamic range for a particular anatomic structure. An auto-contrast map is formed from the localized characteristic curve and inverse output device gray level display function.